This invention relates to ring laser gyros and more particularly to such gyros employing magnetic bias and to transverse Kerr magneto-optic effect mirrors useful in such gyros.
Ring laser gyroscopes using magnetic bias commonly include a laser discharge path confining block to which is coupled or attached a plurality of mirrors forming a closed loop (ring) to define an optical cavity for the propagation of counter-propagating laser beams. Such ring laser gyroscopes operate by combining the counter-propagating beams to form an interference fringe pattern. This pattern displays the beat frequency between the counter-propagating beams caused by rotation of the gyro. An output is developed counting movement of the fringe pattern as it passes a detector during gyro rotation. In triangular ring laser gyros, using the transverse Kerr effect magnetic bias, one mirror has had to be mounted externally of the cavity and is coupled to the laser discharge within the cavity through a Brewster angle window. The window suppresses unwanted S-polarized radiation and thereby force the cavity to resonate in the P-polarized radiation mode. However, the use of the window configuration introduces undesirable scattering and birefringence.
For low input rotation rates the coupling caused by scattering locks the frequency of the counter-propagating waves to each other, resulting in beat frequency disappearance in which the fringes used to detect rotation remain stationary despite gyro rotation. In order to avoid lock-in, nonreciprocal phase shifting with magnetic bias has been proposed by such prior art patents as U.S. Pat. No. 3,851,973 and U.S. Pat. No. 3,927,946 in which the frequency of one of the counter-propagating beams is shifted with respect to the other so that they operate away from the lock-in region. This allows the gyro to be operated at angular rotation rates below the usual lock-in threshold.
U.S. Pat. No. 4,442,414 to Carter suggests a magnetic mirror configuration to achieve magnetic bias in which a magneto-optic film is separated from a saturable magnetic material layer by a multilayer stack of dielectric materials. However, the suggestion suffers from inadequate coupling between the magnetic layers resulting from too wide a separation between the saturable magnetic material and the magneto-optic material. This separation diminishes the ability of the magneto-optic material to be switched rotationally between the two magnetization states required for lock-in avoidance. Carter also suggests that the magnetically saturable layer can be switched by conductors lying on the top side of the magnetically saturable layer with a current return path under the magnetically saturable layer. By using this sandwich construction, the drive field is confined almost entirely between the two conductors on the top which carry the current out and the ground layer which carries the current back. Although this configuration reduces inductance, it also tends to limit the spatial extent of the drive field to the magnetically saturable layer. The spaced-apart magneto-optic layer must be driven by the demagnetizing field from the magnetically saturable layer. In this case the magneto-optic layer would have to be switched by magnetic domain wall motion rather than rotationally. In Carter, the magneto-optic layer is covered by dielectric layers made of magnesium fluoride and zinc sulfide which are ill-suited for contact with He-Ne lasing plasma. There is therefore a need for an improved laser gyro and magnetic mirror for use in an operational ring laser gyro which will overcome the above limitations and disadvantages.